My 21k Training

Sorry to jump in here guys, but since I signed up for a half marathon next May I'm seeking a little advice.

I just started running earlier this year and I'm doing 10km at around 55mins (my PB was 53.30 before I lost the routine in the summer!), so now I'm back to where I was I'll be starting to do the 10km sub 50 minute cycle next week or the following week to push myself a little more. I'll aim for a sub 2 hour half marathon. I know it's not very fast but it'll do for me. So when, and more importantly for me, how do I step up from the sub 50 10km sessions to a half marathon of 1hr 50min training? What are the guidelines for that?

Oh, and on a general note, I found that after falling out of my routine I was stupid and I tried to jump straight back into my old times. Big mistake. Just getting out running for a time period rather than worrying about distance and pace got me back to where I was within a week and there was no stress on the body. It also made jogging fun again for me!